Patent Document 1 discloses an illuminating device using phosphors as light sources. FIG. 1 shows a configuration of the light source unit for the illuminating device.
Referring to FIG. 1, excitation light source 116 includes a plurality of blue laser diodes (LD). Blue excitation light output from excitation light source 116 is transformed into a parallel light flux by collimator lens array 106, then enters dichroic mirror 115. Excitation light source 116 is arranged so as to output S-polarized light to be incident on dichroic mirror 115. Dichroic mirror 115 is arranged so that the blue excitation light forms an incident angle of 45°. Herein, the incident angle is an angle formed between the incident light beam and the normal line at the incident point.
FIG. 2 shows the spectral transmission characteristics of dichroic mirror 115. The vertical axis represents transmittance and the horizontal axis represents wavelength (nm). The solid line shows the spectral transmission characteristics for S-polarization while the broken line represents the spectral transmission characteristics for P-polarization. The cutoff wavelength for S-polarization is 456 nm and the cutoff wavelength for P-polarization is 434 nm. Here, the cutoff wavelength is the wavelength at which the transmittance becomes 50%.
Dichroic mirror 115 has such characteristics for S-polarized light as to transmit light with wavelengths of above 456 nm and reflect light with wavelengths of less than 456 nm and has such characteristics for P-polarized light as to transmit light with wavelengths of above 434 nm and reflect light with wavelengths of less than 434 nm. The wavelength of blue excitation light is, for example 445 nm. The blue excitation light (S-polarization) from excitation light source 116 is reflected by dichroic mirror 115.
The blue excitation light (S-polarization) reflected by dichroic mirror 115 is converted into circular polarization as it passes through quarter-wave plate 108. The blue excitation light (circular polarization) having passed through quarter-wave plate 108 is focused on phosphor layer 103 by two condenser lenses 109a and 109b. 
Phosphor layer 103 is formed on a circular wheel substrate. The wheel substrate is divided into first to third segments with respect to circular direction. Phosphor layer 103 includes a red phosphor area formed in the first segment and a green phosphor area formed in the second segment. The third segment is formed with a reflective mirror. As the wheel substrate rotates, the blue excitation light (circular polarization) is successively radiated on the first to third segments.
In the first segment the phosphor excited by blue excitation light emits red fluorescence. In the second segment the phosphor excited by blue excitation light emits green fluorescence. In the third segments blue excitation light (circular polarization) is reflected by the reflective mirror.
The red fluorescence from the first segment, the green excitation light from the second segment and the blue light (circular polarization) reflected by the reflective mirror of the third segment pass through condenser lenses 109a, 109b and quarter-wave plate 108 successively. In this process, the blue light (circular polarization) from the third segmentation is converted into P-polarization as it passes through quarter wave plate 108. The red fluorescence, green fluorescence and blue light (P-polarization) each pass through dichroic mirror 115. The light (red light, green light and blue light) that passed through dichroic mirror 115 is used as the output light (illuminating light) of the illuminating device.
In general, a metal plate is used as the wheel substrate. Phosphor layer 103 and the reflective mirror are welded to the metal plate by using adhesives.
FIG. 3 is a schematic diagram showing one example of a phosphor wheel in which the phosphor layer and the reflective mirror are welded on the metal plate by use of an adhesive. FIG. 4 is a developed perspective view of the phosphor wheel shown in FIG. 3.
As shown in FIGS. 3 and 4, phosphor layer 103 and reflective mirror 104 have arced configurations and are formed on metal plate 105 as the wheel substrate. Phosphor layer 103 and reflective mirror 104 are welded to metal plate 105 by an adhesive.